1. Field of the Invention
The present invention relates to a physical quantity sensor, such as an acceleration sensor, manufactured using a micro-electro-mechanical system (MEMS) technique.
2. Description of the Related Art
An acceleration sensor formed using a silicon-on-insulator (SOI) substrate includes a movable potion that moves when an acceleration is applied thereto and a detection portion for measuring the displacement of the movable portion. The movable portion and the detection portion are provided on an SOI layer (active layer) positioned above a support substrate.
According to, for example, a known structure for detecting an acceleration applied in a height direction (Z direction), a movable portion is moved in a see-saw manner when an acceleration is applied in the height direction. In the see-saw type structure, the movable portion is supported at a position shifted from the center of gravity of the movable portion. FIGS. 25A and 25B are schematic diagrams illustrating the structure. FIG. 25A is a perspective view of the structure and FIG. 25B is a side view of the structure viewed in the direction shown by the arrow XXVB.
A movable portion 201 includes a first movable portion 203 that extends a long distance in an X1 direction from a support position 202 and a second movable portion 204 that extends a short distance in an X2 direction from the support position 202. When an acceleration is applied in the height direction (Z direction) and the first movable portion 203 tilts downward, the second movable portion 204 tilts upward. Z-axis differential output detection portions 206 and 207, for example, are provided on a support substrate 205 so as to face the first movable portion 203 and the second movable portion 204, respectively.
Japanese Unexamined Patent Application Publication No. 2006-266873 is an example of related art.
However, the Z-axis compatible acceleration sensor illustrated in FIGS. 25A and 25B has the following problems.
That is, to cause the movable portion 201 to adequately tilt in the height direction when an acceleration is applied in the height direction and to increase the displacement of the movable portion 201 in the height direction, the difference between the areas of the first and second movable portions 203 and 204 must be increased. Therefore, it is difficult to reduce the size of the movable portion 201. In addition, to improve the sensor sensitivity, it is necessary to place the Z-axis differential output detection portions 206 and 207 at positions spaced from the support position in the tilting direction, where the first and second movable portions 203 and 204 are moved by large distances.
Therefore, it has been difficult to provide a Z-axis compatible acceleration sensor that is small and that has a high sensitivity.